Voice coil driven positioner for coarse and fine positioning of magnetic head in multi-track tape drive

ABSTRACT

A voice coil driven positioner is provided for positioning the magnetic head of a tape drive relative to the tape of a removable tape cartridge. The head positioner includes a low-mass carriage that supports the head on one end and the voice coil on an opposed second end. A low friction guide mechanism guides the carriage along a precision movement path that carries the head across tracks of a supplied multi-track tape and simultaneously carries the voice coil through a flux field generated by a stator. A movable magnet is attached to the carriage and located within the hollow of the voice coil for reciprocating into a hollow passageway of the stator together with reciprocation of the voice coil into another hollow of the stator. A flux detector is attached to the stator and located compactly within the hollow passageway of the stator for detecting the position of the magnet. A programmable lookup table converts measurement signals output by the flux detector into pre-calibrated, head-position indicating signals. A platform is provided on the carriage for mounting an interferometer prism used during calibration.

This application is a divisional of Ser. No. 08/577,802, filed Dec. 22,1995, which is a continuation of Ser. No. 08/168,716, filed Dec. 15,1993, now abandoned.

BACKGROUND

1. Field of the Invention

The invention relates generally to magnetic recording devices. Theinvention relates more specifically to an apparatus and method forpositioning a magnetic head relative to the tracks of a multi-tracktape.

2a. Cross Reference to Related Applications

The following copending U.S. patent application(s) is/are assigned tothe assignee of the present application, is/are related to the presentapplication and its/their disclosures is/are incorporated herein byreference:

(A) Ser. No. 07/794,999 by Nayak et al. and entitled GEAR DRIVE CARRIAGEAND STEPPER ADJUSTMENT SYSTEM;

(B) Ser. No. 07/926,743 by Nayak et al. and entitled MECHANISMS FOR ACLOSED LOOP HEAD POSITIONER FOR STREAMING TAPE DRIVES.

2b. Cross Reference to Related Patents

The following U.S. patent(s) is/are assigned to the assignee of thepresent application, is/are related to the present application andits/their disclosures is/are incorporated herein by reference:

(A) U.S. Pat. No. 5,191,492 issued Mar. 2, 1993, to Nayak et al. andentitled MECHANISMS FOR A CLOSED LOOP HEAD POSITIONER FOR STREAMING TAPEDRIVES.

3. Description of the Related Art

High-density recording of information on multiple tracks of a magnetictape is well known. Parallel tracks are defined to extend along asubstantially longitudinal direction of an elongated magnetic tape. Amagnetic head is moved in a transverse, lateral direction across thetape surface to bring a read and/or write gap of the head into proximitywith a desired track prior to recording or playback. During a recordingor playback session, the head is expected to remain on track while thetape moves in the longitudinal direction, past the read/write gap.

Accurate positioning of the head's read/write gap to a desired track isimportant. Two basic kinds of head positioners are used for providinghead to track alignment: open-loop and closed-loop.

Open-loop positioners are typically employed for one-time placement of ahead relative to a track and are commonly found in tape systems havingrelatively low track densities. The magnetic head rides on a lead screw.A stepper motor rotates the lead screw and the lead-screw converts therotational motion of the motor into linear movement of the head. Thestepper motor is advanced a fixed number of degrees to shift the headfrom one track to the next prior to reading or writing. The head remainsin a fixed position during the read/write session. No provision is madefor correcting alignment error while the tape moves and information isbeing read from or recorded onto the track.

Closed-loop positioners are typically used for multi-track tapes havingrelatively high track densities and high recording/readback rates. Thetape has a tendency to disadvantageously wander in the lateral directionas it advances in the desired longitudinal direction. This creates anundesirable track-to-head misalignment. If tracks are spaced very closeto one another, the lateral wander can be sufficient to produce anoff-track condition. To overcome this problem, servo signals arepre-recorded onto the tape to mark the position of each track. Aclosed-loop servo system moves the head laterally while searching forthe servo signals of a desired track and thereby brings the head intofine alignment with the desired track. Feed-back is used to continuouslymaintain alignment with the servo signals during the reading or writingof information from/onto the track.

A combination of an open-loop coarse positioner and closed-loop finepositioner has been proposed for positioning a magnetic head relative toa high-density multitrack tape. The above cited U.S. Pat. No. 5,191,492discloses such a combination. A voice coil is attached to the magnetichead for providing closed-loop fine positioning while the head andvoice-coil assembly rides on a lead screw that is driven by astepper-motor to provide coarse positioning.

Such a combination of an open-loop, coarse positioner and a closed-loop,fine positioner suffers from the following drawbacks. Numerous parts areneeded both for individually constructing the coarse and finepositioning subsystems and for connecting the two subsystems together.The cost of manufacture for such a combination is large due to theadditive cost of the individual parts and due to the work involved incombining so many parts. The combined size of the coarse and finepositioning subsystems tends to be disadvantageously large. Integrationof the open-loop and closed-loop servo electronics is complex and raisesstability problems. Track switching time and/or track switching powerconsumption tends to be large due to the combined mass of the coarse andfine positioning subsystems.

SUMMARY OF THE INVENTION

The invention overcomes the above-mentioned problems by using a singlevoice coil to provide both coarse and fine positioning of a magnetichead relative to tracks of a multi-track tape.

A low-mass voice-coil attaches to a movable carriage. A magnetic headalso attaches to the movable carriage. The carriage is guided along aprecision path by a low-friction, motion guidance mechanism. Avoice-coil positioning subsystem is provided for moving the voice-coilto desired, nominal track positions and for thereafter continuouslyaligning the carriage-supported head to track servo signals. Oneembodiment of the positioning subsystem uses an LVDT position detectorfor aligning the head to nominal track positions. Another version uses apair of opposed Hall-effect transducers for positioning the head to thenominal track positions. The LVDT or Hall-effect position detector isplaced within the confines of a cylindrical region whose outside definesa tubular path traveled by the voice-coil. This arrangement provides acompact voice-coil based positioning system.

A structure in accordance with the invention comprises: (a) a movablecarriage supporting a magnetic head at one end of the carriage and avoice coil at a second end of the carriage; (b) a guide rail for guidingthe movable carriage along a precision guide path that runs parallel andlateral to a tape surface; (c) a carriage position detecting means fordetecting the position of the carriage relative to a reference; (d) astator assembly for generating a stationary flux field passing throughthe voice-coil; and (e) voice-coil drive means for driving a currentthrough the voice-coil to thereby propel the voice-coil and attachedhead to a desired position, the voice-coil drive means including anominal track positioner subsystem for moving the head to predefinednominal track positions and a closed-loop servo system for thereaftermoving the head into fine alignment with servo signals of a desiredtrack.

A method in accordance with the invention comprises the steps of: (a)providing a voice coil reciprocally disposed in a magnetic statorstructure, the voice coil moving relative to the stator structure inresponse to the application of electrical current through the voicecoil; (b) providing a programmable calibration means for measuringmovement of the voice coil relative to the stator structure, the outputof the calibration means being defined by a programmable table look-upmeans; (c) removably attaching a precision measurement means to thevoice coil for precisely measuring the position of the voice coilrelative to the stator structure; (d) applying current to the voice coilto move the voice coil to a precision position as determined by theprecision measurement means; (e) programming the programmable tablelook-up means to output a signal representing said position while saidcurrent is applied to the voice coil.

BRIEF DESCRIPTION OF THE DRAWINGS

The below detailed description makes reference to the accompanyingdrawings, in which:

FIG. 1 is a cross-sectional side view showing a voice coil drivenpositioner, and a schematic of accompanying control electronics, inaccordance with the invention;

FIG. 2 is a top plan view showing the voice coil driven positioner of anembodiment of FIG. 1, engaged with a removable tape cartridge;

FIG. 3A is a side view of a spring-loaded, movable guide rail;

FIG. 3B is a top plan view of part of the structure shown in FIG. 3A;

FIG. 4A is a cross-sectional side view showing a voice coil drivenpositioner including a Hall-effect position detector;

FIG. 4B is a cross-sectional side view showing a voice coil drivenpositioner including an optical position detector;

FIG. 5A is a top plan view showing an alternate embodiment for thecarriage guide means;

FIG. 5B is a cross-sectional top view showing details of the carriageslide mechanism shown in FIG. 5A.

DETAILED DESCRIPTION

Referring to FIG. 1, a sectional side view is shown of a voice coildriven, head-positioning system 100 in accordance with the invention. Itis to be understood that this cross sectional view is not to scale. An"X" axis extends horizontally across FIG. 1 from left to right. A "Z"axis extends vertically from bottom to top. A "Y" axis is understood toextend out perpendicularly from the plane defined by the X and Z axes.

The head-positioning system 100 is formed on a diecast frame 110. Frame110 is preferably made of a nonmagnetic, rigid and thermally stablematerial, which is preferably a metal alloy such as Aluminum Alloy 380.The frame 110 has a relatively flat, support surface 111 defined thereonfor supporting a stator assembly 120. Surface 111 runs parallel to theXY plane. (Part of frame 110 is cut-away in FIG. 1 to show connectionwires 151 and 152.)

The frame 110 includes a die-cast alignment feature 112 projectingvertically from a first part of the stator support surface 111. Aguide-rail mounting hole 113 is bored perpendicularly into a second partof support surface 111 at a predetermined distance "A" away from a rightwall of the alignment feature 112, as measured along the X axis.Distance A is greater than an outer wall-to-wall dimension "B" of abelow-described stator assembly 120 and has a precision of 0.0005"(one-half thousandth of an inch) or better.

A carriage guiding rail 130, made of a rigid, preferably nonmagneticmaterial such as stainless steel is fixedly mounted into and centeredaxially over the guide-rail mounting hole 113 by press fitting,threading or other appropriate means. The material of the fixed guiderail 130 is preferably machined to define a smooth cylindrical shafthaving an outside diameter of 0.156 inch at a precision of 0.0002" orbetter.

A movable carriage 160, having six low-friction roller wheels 161-166protruding therefrom, engages with and rides along the fixed guide rail130 as will be explained below. The purpose of the fixed guide rail 130is to guide the movable carriage 160 along a precision path that runs inthe Z direction. The purpose of the six low-friction roller wheels161-166 is to serve as a low-friction means for riding along the fixedguide rail 130; and along a below-described, spring-biased, movableguide rail 135; with substantially small friction so as to therebyminimize a corresponding friction loading on a below-describedvoice-coil 140 when the voice coil propels the moveable carriage 160.

Stator assembly 120 is placed on the stator support surface 111 and isabutted against the right wall of alignment feature 112 so as to beprecisely positioned relative to the position of the fixed guide rail130. The stator assembly 120 has a machined, outer wall-to-walldimension B that is less than distance A, as mentioned earlier. Theouter wall-to-wall dimension B is machined to a precision of 0.00051" orbetter so that the dimensional difference, A-B, has a precision of0.001" or better.

Stator assembly 120 preferably has a symmetrical tubular shape and morepreferably a cylindrical shape with a coaxial cylindrical hollow (122)defined centrally therethrough. (In an alternate embodiment, the XYcross section of stator assembly 120 has a rectangular shape withrounded corners, not shown.) In the case where stator assembly 120 iscylindrical, the outer surface 121 of stator assembly 120 preferably hasan O.D. (outside diameter) of 1.000 inches. Internal components of thestator assembly 120 are symmetrically disposed relative to the statorouter surface 121 and positioned at a tolerance of 0.001" or betterrelative to the stator outer surface 121.

An LVDT coil assembly 123, composed of a primary coil 123a and twosecondary coils 123b and 123c, is secured centrally within the statorassembly 120. A core passageway 122 extends centrally through the LVDTcoil assembly 123 in the Z direction for allowing free movementtherethrough of an LVDT core piece 123d. The operation of the LVDT coilassembly 123 will be detailed below. For now, it is sufficient tounderstand that the LVDT coil assembly 123 constitutes a centrallylocated position measuring means that is used for measuring the positionalong the Z axis of the LVDT core piece 123d relative to the primary andsecondary LVDT coils 123a-123c. The combination of the LVDT core piece123d and a below-described core-support pedestal 167a constitutes alow-mass "dip stick" which reciprocates into core passageway 122 inunison with the reciprocation of a below described voice-coil 140 into amagnetic gap 125 of stator assembly 120.

An inner pole piece 124 surrounds and fastens to the outside of the LVDTcoil assembly 123. The inner pole piece 124 is made of a magneticallyconductive material such as steel. Pole piece 124 is used to conductmagnetic flux produced by a below-described set of permanent magnets126. An appropriate epoxy adhesive may be used for binding the outersurface of the LVDT coil assembly 123 to the inner surface of the innerpole piece 124 or the LVDT coil assembly 123 may be press fitted intothe cylindrical hollow of the inner pole piece 124. The inner pole piece124 is preferably of a one-piece tubular construction.

The LVDT coil assembly 123 includes its own magnetic housing forretaining flux produced by the LVDT coils 123. This housing structureincludes a small bonding spacer (not shown) that is 0.005 inch thick andcentrally located about the middle of the outer surface of the LVDThousing for adhesive or press-fit attaching the housing at a singularpoint to the inner diameter of inner pole piece 124. An adhesive such asLoctite Blackmax 380™ may be used for adhesively attaching the bondingspacer to the inner pole piece 124. A gap of 0.005 inch (not shown) isestablished between the remainder of the outer surface of the LVDT coilassembly 123 and the inner diameter of pole piece 124 so that theremainder of the LVDT assembly 123 is free to expand equally above andbelow the centrally located bonding spacer as temperature changes.

In the preferred case where LVDT coil assembly 123 is cylindrical inshape, the LVDT coil assembly 123 has an inner diameter (I.D.) of 0.160inches and an O.D. of 0.300 inches while the inner pole piece 124 has anI.D. of 0.310 inches and an O.D. of 0.550 inches.

An outer pole piece 127, preferably made of the same magneticallyconducting material (steel) as the inner pole piece 124, surrounds andbonds to the inner pole piece 124. The cross section of the outer polepiece 127 resembles two side-by-side capital L's, with the capital L onthe left being in normal orientation and the capital L on the rightbeing in a facing mirror image orientation. The base legs (127a) of theforward and mirror image L's are joined to the outer surface of theinner pole piece 124 either by press fitting (with an interference fitof 0.0005 inch) or by bonding with an appropriate bonding material. Theouter pole piece 127 is preferably of a one-piece tubular construction.The inner and outer pole pieces, 124 and 127, can be each formed assingular cast pieces and then joined together or, alternatively, theycan be cast together as a continuous unitary piece.

The combination of inner pole piece 124 and outer pole piece 127 definesin the XZ plane, two spaced apart U-shaped magnetic-flux conductors (oryokes) through which flux 126a of a soon-described set of permanentmagnets 126 flows. In the preferred case where inner pole piece 124 iscylindrical in shape, the outer cylindrical portion of the outer polepiece 127 has an inner diameter (I.D.) of 0.900 inches and an O.D. of1.000 inches (the latter is dimension B) while the base 127a of theouter pole piece 127 has a thickness of 0.100 inches.

A set of permanent magnets 126 are bonded to the upper inner wall ofouter pole piece 127. The permanent magnets 126 are oriented to havetheir respective north poles facing flush against the inner wall ofouter pole piece 127 and their respective south poles facing inwardlytowards the center of the stator assembly 120. In the preferred casewhere outer pole piece 127 is cylindrical in shape, the permanentmagnets 126 are provided as a plurality of arc segments mounted to theinner wall of outer pole piece 127. By way of example, the permanentmagnets 126 can be provided as a plurality of four 90° arc segments orsix 60° arc segments. The preferred material for permanent magnets 126is Neodymium or a like high-magnetic strength material. Flux lines 126aflow radially and uniformly from the south poles to the outer wall ofinner pole piece 124.

The permanent magnets 126 are preferably positioned a small distance C(e.g., 0.020 inch) below the top edges of outer and inner pole pieces127 and 124 so that fringe south-to-north flux flows (not shown) arecaptured by the top edge of outer pole piece 127 and returned to thenorth poles of the permanent magnets 126. The protruding top edge ofouter pole piece 127 is also used to direct fringe flux-flows of asoon-described voice-coil 140 away from a later-described magnetic head170.

A small permanent magnet 128 that serves as part of a limit switch128/148, is positioned at the top of the stator outer surface 121 withthe north (N) pole of magnet 128 facing up in the Z direction. Amagnetic shield 129 having a half-cup shape surrounds and supports theopposed south (S) pole portion of reference magnet 128. The magneticshield 129 directs the flux lines of small magnet 128 away from thesystem's magnetic head 170.

The thickness and material of outer pole piece 127 should be selectedrelative to the strength of permanent magnets 126 (and the smallreference magnet 128) to prevent magnetic saturation of the outer polepiece 127 and thereby assure that fringe flux flows will be captured bypiece 127 and directed away from head 170. Although not shown, anappropriate mu-shield may be placed around head 170 to shield itsread/write gaps from stray magnetic fields. It has been found inpractice that the distance between magnetic head 170 and sources ofstray flux such as the voice-coil motor 120/140 or reference magnet 128is often sufficiently large to reduce stray field strength to anacceptable level (e.g., 5 gauss). In such a case the mu-shield is notnecessary.

A magnetic gap 125 is defined between the inwardly facing south poles ofthe permanent magnets 126 and the upper outer wall of inner pole piece124. The magnetic gap 125 is sized to provide clearance for thebelow-described voice-coil 140 to move through the gap in the Zdirection. A nominal clearance 125a of 0.010" or more is preferred aboutthe voice-coil 140 in all directions of the X and Y plane.

In the preferred case where the inner and outer pole pieces, 124 and127, are both cylindrical in shape, and coaxial, each permanent magnet126 has a thickness in the XY plane of 0.060, thereby defining the innerdiameter (I.D.) of the set of permanent magnets 126 as 0.780 inch. Thevoice coil has a thickness of approximately 0.075 inch. (Thecross-sectional thickness of the voice-coil 140 is equal to the radialwidth of the magnetic gap 125 less twice the nominal clearance 125a.)Since the outer pole piece 127 has an O.D. of 1.00 inches and an I.D. of0.90 inch, while the inner pole piece 124 has an O.D. of 0.550 inches,the nominal clearance dimension 125a works out to approximately 0.020inch about each of the inner and outer walls of the voice-coil 140.

The nominal clearance 125a between the boundaries of magnetic gap 125and voice-coil 140 is used to take up dimensional deviations in theposition of the voice-coil 140 and a below-described core-supportpedestal 167a relative to the stator assembly 120. The dimensionaldeviations can come from a number of sources, including but not limitedto, thermal expansion of parts of the carriage 160 such as asoon-described left carriage beam 167, error in machining the O.D. ofouter pole piece 127, error in placing guide-rail mounting hole 113relative to alignment feature 112, and error in the machining ormounting of fixed guide rail 130.

If desired, base protrusions 120a having slotted alignment holes 120bmay be provided at the outer base portion of stator assembly 120 foraligning the stator assembly 120 relative to fixed guide rail 130 at thetime that the stator assembly 120 is secured to the support frame 110.In such a case, alignment feature 112 can be dispensed with, or usedonly for crude alignment. In the preferred embodiment, however,alignment feature 112 is used for precisely aligning the stator assembly120 relative to fixed guide rail 130 in one step and an epoxy adhesiveis deposited as a uniformly thick layer under the outer pole piece 127for fixedly securing the stator assembly 120 to support surface 111. Thelatter approach reduces manufacturing costs. Other, equivalent fasteningmeans can of course be used to secure stator assembly 120 to frame 110.

Voice-coil 140 cuts through the flux lines 126a of magnetic gap 125 suchthat an upwardly or downwardly directed force F is induced against thevoice-coil 140 when an electrical current I_(C) of correspondingpositive or negative polarity flows through voice-coil 140. Thevoice-coil 140 is tube shaped and has an interior hollow 140a thatallows the body of the voice-coil 140 to freely reciprocate over theouter boundary of inner pole piece 124. The voice-coil 140 mounts to thebottom of a left carriage beam 167 provided on carriage 160 and therebypropels the carriage 160 up or down in accordance with the magnitude andpolarity of current I_(C) passing through voice-coil 140. If theelectrical ends of coil 140 are shorted together to maintain zero voltsacross the ends, a well-known braking effect develops.

Carriage 160 is made of a nonmagnetic, rigid, preferably low-massmaterial such as aluminum. The carriage includes a carriage center piece168 (from which the earlier mentioned wheels 161-166 protrude), a leftcarriage beam 167 extending integrally from the top of a left side ofthe carriage center piece 168, and a right carriage beam 169 extendingintegrally from the bottom of an opposed right side of the carriagecenter piece 168. Left and right carriage beams 167 and 169 aredimensioned so that center portion 168 defines the center of gravity ofcarriage 160 and equal amounts of mass load the left roller wheels 161,163, 165 and the counterpoised right roller wheels 162, 164, 166. Thebalanced loading masses include the mass of magnetic head 170 and themass of voice coil 140 plus LVDT core piece 123d. The moments of theopposed left and right masses are preferably also balanced.

As seen in FIG. 1, a magnetic head 170 is adhesively mounted to a topsurface of the right carriage beam 169. A removable tape cartridge 180that contains a magnetic tape 190 is brought into engagement with theframe 110 such that a low-friction recording surface 191 of the tape 190is brought to bear against a parallel read/write face 171 of themagnetic head 170. Tape cartridge 180 aligns to the frame 110 of thehead-positioning system 100 so as to hold the tape's recording surface191 in a generally perpendicular orientation relative to the statorsupport surface 111 of frame 110 and a generally parallel orientationrelative to the fixed guide rail 130. Tape 190 is elongated generally inthe Y direction and flexes for winding around supply and take-up reels,not shown. The tape also flexes to make conforming contact with theread/write face 171 of head 170.

The read/write face 171 preferably has a stack of two or more read gapsand two or more write gaps arranged in opposition to permit read beforewrite on a given track either when the tape 190 moves in a forwarddirection or a reverse direction.

Carriage 160 moves the magnetic head 170 up or down in the Z directionwhile maintaining engagement of the head read/write face 171 with thetape's recording surface 191. The magnetic head 170 is moved so that adesired one of the forward-reverse read/write gap pairs, (R)W or (W)R,is brought into alignment with a desired one of plural tracks 192defined on the tape's recording surface 191. While not shown, it isunderstood that servo signals are pre-recorded on the tape's recordingsurface 191 to define the positions of the plural tracks 192.

The fixed guide rail 130 is used to limit the degrees of freedom of thecarriage 160 and to thereby define a precision path along which themagnetic head 170 moves as voice-coil 140 propels the carriage 160 upand down. Four of the low-friction roller wheels, 161-164, are rotatablymounted on a front side of carriage center piece 168 and oriented forriding stably and with minimal drag against a back surface of the fixedguide rail 130. (See the top plan view of FIG. 2.)

As seen in FIG. 1, the first and second roller wheels 161 and 162 arepositioned at a lower portion of carriage center piece 168 while thethird and fourth roller wheels 163 and 164 are positioned incounter-symmetry at an upper portion of carriage center piece 168 so asto hold the carriage center piece 168 in parallel and stable alignmentwith a back surface of the fixed guide rail 130. Right carriage beam 169extends rigidly from the bottom of the carriage center piece 168 so asto hold the head's read/write surface 171 parallel to the travel path ofthe carriage center piece 168. Left carriage beam 167 extends rigidlyfrom the top of the carriage center piece 168 so as to hold a centralaxis the voice-coil 140 parallel to the travel path of the carriagecenter piece 168.

The fifth and sixth roller wheels 165 and 166 are rotatably mounted on aback side of carriage center piece 168 and oriented for riding stablyand with low-friction against a spring-biased, movable guide rail 135(shown in FIGS. 2 and 3). The fifth and sixth roller wheels 165 and 166are positioned symmetrically between the lower wheels 161-162 and theupper wheels 163-164 in order to apply equal subcomponents of a biasforce G (FIG. 2) supplied to the fifth and sixth roller wheels 165 and166 from the spring-biased, movable guide rail 135 shown in FIG. 2.

Roller wheels 161-166 are preferably all identical and each formed of alow-friction bearing such as a 0.156 O.D. type available from NMBCorporation of Chatsworth, Calif.

Referring to FIG. 2, a top plan view of a preferred embodiment 100'having the cross section of FIG. 1 is shown. Although like referencenumbers are used to refer to elements of FIG. 2 that have likecounterparts in FIG. 1, it is to be understood that other top-viewembodiments of the cross section of FIG. 1 are possible.

In FIG. 2, the X axis runs from the axial center of cylindrical statorassembly 120 to and through the axial center of fixed guide rail 130.Tape 190 extends in a direction roughly 50° counterclockwise of the Xdirection. A cartridge cover 185 of tape cartridge 180 is opened toexpose the tape's recording surface 191. The exposed recording surface191 is brought to bear against the read/write face 171 of magnetic head170 as the tape cartridge 180 is inserted into the recording mechanism.

Carriage center piece 168 has a rectangular top-view profile withbeveled corners as shown. The elongated center axis of this rectangularshape runs in the Y direction, perpendicular to the X axis. Pins arepress-fitted into the beveled corner faces of carriage center piece 168to support roller wheels 161-166. Roller wheels 161-164 make tangentialcontact with the back surface of fixed guide rail 130 while rollerwheels 165-166 make tangential contact with the front surface of thespring-biased, movable guide rail 135. The movable guide rail 135 hasthe same O.D. as fixed guide rail 130 and is mounted vertically on pivotarm 136. The pivot arm 136 pivots about pivot shaft 137 so as to bringthe movable guide rail 135 into engagement with roller wheels 165-166while the axial axis (Z direction axis) of the spring-biased, movableguide rail 135 aligns substantially with the elongated center axis ofthe carriage center piece 168 to a precision of 0.001" or better. Theorientation of carriage 160 is thereby set in the X by Y plane of FIG. 2using the bias force G provided by the spring-biased, movable guide rail135.

FIG. 3A shows a side cross-sectional view of a preferred spring-loadedmechanism for applying biasing force G to the movable guide rail 135.Pivot shaft 137 is press fit into a bore hole 117 of frame 110 at apredetermined location relative to the fixed guide-rail mounting hole113. Positioning precision should be to within 0.001" or better. Twosubstantially identical spiral springs, 141 and 142, are placed inmirror image orientation about pivot shaft 137. The springs 141-142apply biasing force G to the center of movable guide rail 135 by way ofrespective first spring ends, as shown. The other ends of springs141-142 rest against respective top and middle portions of a spring-restpost 139. The spring-rest post 139 is mounted vertically into a threadedbore hole 119 of frame 110.

Pivot arm 136 is pivotally mounted at a top portion of pivot shaft 137and rests on top of the biasing springs, 141 and 142. A substantiallysimilar, lower pivot arm 138 is pivotally mounted at a lower portion ofpivot shaft 137, below the biasing springs, 141 and 142, but resting ontop of a support bushing 143. Spring-biased, movable guide rail 135 issupported between the extreme ends of pivot arm 136 and lower pivot arm138, to provide a total travel distance D thereon for the two rollerwheels 165-166. Travel distance D should be sufficiently large andappropriately positioned to allow the read/write gaps of magnetic head170 to begin their Z-direction travel below the lower edge of the tape'srecording surface 191 and to cover the full lateral extent of the tape'srecording surface 191.

Referring again to FIG. 2, the magnetic head 170 is mounted on carriage160 at the same approximately 50° angle relative to axis X as that ofthe tape 190 so that the head read/write face 171 engages relativelyflush with the tape surface 191. Excessive shocks that may be applied tothe free-floating carriage 160 are absorbed by rotation of thespring-loaded pivot arm 136 out of its nominal position. Thespring-provided loading force G is set to hold movable guide rail 135 inits nominal position for normal shock and vibration.

The alignment feature 112 of FIG. 2 preferably has a semi-cylindricalshape as shown in FIG. 2. During assembly, the spring-biased, movableguide rail 135 is retracted away from the region occupied carriagecenter piece 168. The combination of stator assembly 120 and carriage160 is positioned by abutting the stator outer surface 121 againstalignment feature 112 and further abutting the four roller wheels161-164 against fixed guide rail 130. The movable guide rail 135 isreleased to engage roller wheels 165-166. The ability of the carriage160 to travel freely up and down along fixed guide rail 130 is testedbefore the adhesive that bonds stator assembly 120 to frame 110 sets.

Referring again to FIG. 1, the operation of the LVDT coil assembly 123is now explained in detail.

A first step in bringing a read/write gap ((W)R or R(W)) into alignmentwith the servo signals of a desired track is to move the read/write gapin open-loop fashion to a nominal track position that coarsely alignswith the desired track. The LVDT coil assembly 123 is used for suchcoarse alignment.

An excitational AC signal 151 is applied to the primary coil 123a of theLVDT coil assembly 123 from an LVDT control module 150. LVDT core piece123d couples the excitational AC signal to secondary coils 123b and123c. The magnitude of excitational energy coupled to each of secondarycoils 123b and 123c varies with the position of LVDT core piece 123drelative to each of the secondary coils 123b and 123c.

The secondary coils 123b and 123c are series connected in a 180° phaserelation so as to produce a null, secondary output signal 152 when theLVDT core piece 123d is centered relative to the secondary coils 123band 123c. The magnitude of the secondary output signal 152 varies in agenerally linear fashion as the LVDT core piece 123d is moved up ordown, away from the center position. The polarity of the secondaryoutput signal 152 indicates whether the LVDT core piece 123d has movedeither above or below the center position.

The LVDT control module 150 outputs an analog, DC voltage, V_(P)representing the position of the LVDT core piece 123d relative to thesecondary coils 123b and 123c. LVDT transducer systems and more detaileddescriptions thereof may be obtained from Lucas Schaevitz, Inc. ofPennsauken, N.J.

A first analog-to-digital converter (ADC) 153 converts the analog outputvoltage, V_(P), of the LVDT control module 150 into a first digitalindex signal 155a. This first digital index signal 155a serves as partor the whole of a look-up address signal supplied to a look-up tablemodule 155.

Look-up table module 155 is preferably a programmable-read-only memoryunit (PROM) that is programmed in accordance with a below describedcalibration method to produce a digital, head-position indicating signal156. The head position indicating signal 156 is binary coded torepresent the position of magnetic head 170 relative to frame 110 to adesired precision and accuracy. It is to be understood of course, thatother, like code-conversion means can be used for converting the code ofthe first digital index signal 155a (optionally combined with asoon-described second digital index signal 155b) into the code of thehead-position representing signal 156.

Head position indicating signal 156 is supplied to a first input of avoice-coil drive module 159. The voice-coil drive module 159 has asecond input which receives an externally supplied, digital code signal157 representing a desired, nominal track-position. A feed-back circuit(not shown) of the voice-coil drive module 159 outputs voice-coilcurrent I_(C) to voice-coil 140 so as to bring the position representedby the head position indicating signal 156 into convergence with thedesired head position that is represented by the nominal track-positionsignal 157.

Voice-coil drive module 159 has a third input which receives track servosignals 158 picked up by the active read gap ((R) or R) of magnetic head170. A fourth input of the voice-coil drive module 159 receives alimit-indicating signal 149 from a below described Hall-effect detector148 of a limit switch 128/148.

The feed-back circuit (not shown) of voice-coil drive module 159 remainsactive until it reduces the error between the head position indicatingsignal 156 and the nominal track-position signal 157 to a predefined,acceptably small level. Then a closed-loop servo circuit (not shown) ofvoice-coil drive module 159 takes control. The closed-loop servo circuitadjusts the voice-coil current I_(C) in response to the received servosignals 158 to bring head 170 into fine alignment with the desired trackand to continuously maintain such alignment as long as the current trackcontinues to be the desired track. When a switch over to a new track isrequested, a corresponding new nominal track-position signal 157 issupplied and the feed-back circuit (not shown) of voice-coil drivemodule 159 is reactivated. If the tape does not have track-centeringservo signals recorded thereon, closed-loop servo circuit (not shown) ofvoice-coil drive module 159 is not activated and the feed-back circuit(not shown) of voice-coil drive module 159 remains active to minimizeerror between the head position indicating signal 156 and the nominaltrack-position signal 157.

Thermal expansion of the material forming carriage 160, and particularlyof the core-support pedestal 167a, becomes a problem when open-looppositioning for very-closely spaced tracks is desired. A thermistor 146or like temperature sensor is placed in a hollow 167c formed at a topportion of left carriage beam 167, preferably near the core-supportpedestal 167a. The thermistor 146 is used for sensing the temperature ofthe head-positioning system 100 in general and more specifically, of thecore-support pedestal 167a. An analog voltage, V_(T), representing thetemperature at hollow 167c develops across thermistor 146 and is carriedby fine wires 147 over the top of carriage 160 to a flexible cable 145that extends off of the carriage center piece 168. The flexible cable145 flexibly connects electrical terminals at the top of the carriagecenter piece 168 to electronic control modules such as 159 and 154 thatare mounted on frame 110. Electronic control modules such as 159, 154are preferably mounted on frame 110 in order to minimize the mass ofmovable carriage 160 and its supported elements, e.g., 140 and 170.Flexible cable 145 should be sufficiently flexible so that it appliesnegligible drag to the movement of carriage 160.

The analog thermistor voltage, V_(T), couples by way of the flexiblecable 145 to a second analog-to-digital converter (ADC) 154. The secondanalog-to-digital converter (ADC) 154 converts V_(T) into a seconddigital index signal 155b which is optionally combined with firstdigital index signal 155a to define the address input signal that isapplied to look-up table module 155. If temperature compensation is notdesired, the second digital index signal 155b is fixed at zero andthermistor 146 is not included in the system.

Flexible cable 145 also carries the voice-coil current I_(C) fromvoice-coil drive module 159 to terminal wires 144 of voice-coil 140. Athrough-hole 167b is provided in carriage 160 communicating between thebottom and top of the left carriage beam 167 for passing the voice-coilterminal wires 144 from under beam 167 to its top. wires 144 run on topof beam 167 to the connection points provided at the top of carriagecenter piece 168 to flexible cable 145.

The flexible cable 145 further carries a reference position signal 149(also referred to as a limit-indicating signal 149) from a Hall-effectdetector 148 of the earlier mentioned limit switch 128/148 to the fourthinput of voice-coil drive module 159. The Hall-effect detector 148 ispositioned in a hollow 167d of left carriage beam 167 and alignedvertically above the small reference magnet 128 that is provided on thestator outer surface 121. When the magnetic field density of referencemagnet 128 reaches a critical strength, as the bottom of left carriagebeam 167 approaches closer and closer to the top of stator assembly 120,the reference position signal 149 output by the Hall-effect detector 148switches from a logic low to logic high level. This reference positionsignal 149 indicates a limit position of the carriage 160 where leftcarriage beam 167 is about to hit against the top of outer pole piece127. This indication of the limit position 149 is fed to the voice-coildrive module 159 and used by module 159 to avoid driving the carriage160 into collision with the stator assembly 120.

The UGN3503™ is an example of a digital Hall-effect detector that can beused for implementing limit detector 148. The UGN3503™ is an integratedcircuit which combines in one package, a Hall-effect element and atemperature stabilized comparator/amplifier and a Schmitt trigger outputstage having a predefined amount of hysteresis. The UGN3503™ isavailable from Allegro Microsystems, Inc. of Worcester, Mass.

Calibration and programming of the look-up table module 155 proceedsafter completion of the assembly of head-positioning system 100, asfollows. The head-positioning system 100 is placed in atemperature-stabilized environment and the temperature is set to a firstof plural subdivisions of an expected range of operating temperatures.

A reference platform 175 is provided on the right carriage beam 169 forremovably attaching an interferometer mirror (not shown) or morepreferably an interferometer prism (not shown) to the movable carriage160. A laser interferometer system, that includes the interferometerprism/mirror (not shown) removably clipped onto the reference platform175, is used to precisely measure the position of the magnetic head 170relative to the frame 110. The output of look-up table module 155 isdecoupled from the first input (156) of the voice-coil drive module 159at this time. The LVDT coil assembly 123 and LVDT control module 150remain active, however.

The nominal track-position signal 157 at the second input of thevoice-coil drive module 159 is set to an appropriate level (analog ordigital) that indicates a desired nominal track position. The decoupledfirst input (156) of the voice-coil drive module 159 is then adjusted(with an external supply rather than from the output of look-up tablemodule 155) to produce a voice-coil current I_(C) that brings themagnetic head 170 to the desired nominal track position based onmeasurements of the interferometer at the set temperature.

For each nominal track position in the set of all nominal trackpositions, the corresponding input (156) of the voice-coil drive module159 that produces the desired positioning of the magnetic head 170 iswritten into look-up table module 155 at the address then indicated bythe first and second digital index signals, 155a and 155b.

After data is collected for all nominal track positions at the firsttemperature, the temperature is set to another of the pluralsubdivisions of the expected range of operating temperatures, and theabove calibration process is repeated. This step is repeated untilcalibration has been performed for all of the plural subdivisions of theexpected range of operating temperatures.

After calibration is completed for all nominal track positions and allsubdivisions of the expected range of operating temperatures, remainingentries of look-up table module 155, if any, are filled byinterpolation. A detailed description of this process may be found inthe above cited patent application, Ser. No. 07/794,999 entitled GEARDRIVE CARRIAGE AND STEPPER ADJUSTMENT SYSTEM.

Once interpolation is complete, look-up table module 155 will store acustom translation algorithm for converting each given combination offirst and second digital index signals 155a and 155b (LVDT output andthermistor output) of the specific head-positioning system 100 into acorresponding and calibrated, head position indicating signal 156. Then,when the output of look-up table module 155 is reconnected to firstinput (156) of the voice-coil drive module 159, the head positionindicating signal 156 will provide a calibrated indication of the actualhead position. The feed-back circuit (not shown) of the voice-coil drivemodule 159 can then be used to output a voice-coil current I_(C) thataccurately brings the position of the head 170, as indicated by the headposition indicating signal 156, into convergence with the desired headposition that is requested by the nominal track-position signal 157.

FIG. 4 shows an alternate embodiment 400 in which another form ofposition indicating means 423 is provided within the hollow 422 of thetubular shape of the stator assembly 120' for indicating the position ofthe magnetic head 170 relative to a desired nominal track position. Allother elements of the head positioning system remain substantially thesame as those in FIG. 1 and are thus not shown. Reference numerals inthe "400" series are used to denote elements having like counterpartsnumbered in the "100" series in FIG. 1.

The alternate position indicating means 423 comprises a pair ofHall-effect detectors 423a and 423b mounted at opposed ends of thehollow passageway 422. Like the LVDT coil assembly 123, theHall-effect-based position indicating means 423 is positioned in theregion that fills the interior of a tubular path taken by voice-coil140. This arrangement provides a compact voice-coil motor withintegrated position sensing means. A movable, permanent bar magnet 423cmoves up and down within the hollow passageway 422. The south pole (S)of magnet 423c faces up in the Z direction while the north pole (N)faces down. The detected strengths of the magnetic fields generated bythe movable permanent magnet 423c increase or decrease as magnet 423crespectively approaches or moves away from a given one of Hall-effectdetectors 423a and 423b. The voltage across a Hall-effect element ineach detector varies roughly in proportion to magnetic field strength.The UGN3503™ is an example of a linear Hall-effect detector that can beused for implementing Hall-effect detectors 423a and 423b. The UGN3503™is an integrated circuit which combines in one package a Hall-effectelement and a linear, temperature stabilized amplifier; and is availablefrom Allegro Microsystems, Inc. of Worcester, Mass.

An L-shaped support bracket 467a that is made of a nonmagnetic materialsuch as aluminum connects the movable permanent magnet 423c to thecarriage 160 so that magnet 423c moves together with the carriage 160and thereby indicates the position of the carriage by the strength ofthe magnetic field radiated to the Hall-effect detectors 423a and 423bfrom the poles of movable permanent magnet 423c. Thermistor 146' is usedfor measuring the temperature of support bracket 467a and is used as aninput to the look-up table module 155 (not shown) for compensating forthermal expansion of the support bracket 467a.

Magnet 423c should be of a low-weight, high-field strength formulationsuch as Neodymium so that the overall mass of the movable carriage 160is not significantly increased by the mass of the magnet 423c. Innerpole piece 124 acts as a magnetic shield for diverting the flux lines ofthe movable permanent magnet 423c away from magnetic head 170. Ifdesired, an additional mu-shield 423d can be formed aboutHall-effect-based position indicating means 423 as shown.

The output voltages of Hall-effect detectors 423a and 423b are denotedrespectively as V_(A) and V_(B). These voltages are output on wires 452and combined in accordance with below equation Eq. 1 to define aposition voltage V_(P) that indicates the position of thecarriage-supported, magnetic head 170.

    V.sub.P =(V.sub.A -V.sub.B)/(V.sub.A +V.sub.B)             (Eq. 1)

Note that V_(P) nulls out when the movable permanent magnet 423c isapproximately midway between Hall-effect detectors 423a and 423b. Thedenominator (V_(A) +V_(B)) of Eq. 1 provides enhanced sensitivity nearthe midpoint of the travel of movable permanent magnet 423c, where thesum V_(A) +V_(B) tends to be at a minimum. The ratiometric definition ofposition, as given by Eq. 1, eliminates dependency on the absolutevalues of respective voltages V_(A) and V_(B). V_(P) is unique for eachposition of the movable permanent magnet 423c.

The look-up table calibration process described for the LVDT-basedposition indicating assembly 123 is also applicable to theHall-effect-based position indicating means 423 and as such, will not bedescribed again here.

Those skilled in the art of position measurement will recognize from theabove that other forms of position determining means can be providedwithin the hollow 422 of the tubular shape of the voice-coil 140 forindicating the position of the magnetic head 170 relative to a desirednominal track position. It is desirable that the movable portion thatconnects to carriage 160 should be of low mass and the overall positiondetermining means should be compact in size.

By way of further example, FIG. 4B shows how an optical positiondetermining means can be formed using an optical scale 467a' thatreciprocates vertically within hollow passageway 422 of stator assembly120 and a scale reading means (e.g., one or more LED's and photodiodes)423' fitted in the hollow. Scale 467' is preferably a light weightoptical ruler having quadrature or other optical patterns definedthereon. A fixed set of one or more LED's and photodiodes are providedin scale reading means 423' and fitted in the hollow 422 forilluminating and reading the quadrature or other optical patternsdefined on the scale 467a'.

Those skilled in the art of motor design will recognize from the abovethat the combination of a voice-coil 140 and interiorly positioned means(123d or 423c or 467a') for determining the position of the voice-coilprovides a compact motor that can be used for many applications otherthan the positioning of a magnetic recording head 170 relative to atape.

Referring to FIG. 5A, a top view is shown of an alternate guidemechanism 500 for defining the paths traveled by the carriage supportedvoice-coil 140 and head 170. A slide member 568 is fixedly fastened byscrews or other means to a center portion of a carriage 160'. In thisparticular embodiment, carriage 160' does not have roller wheels 161-166protruding from its center piece. Slide member 568 is made of analuminum alloy having a certain degree of flexure (e.g, Aluminum 6061T-6) and machined to a fine smoothness.

A stationary member 530 that has a C-shape in the XY plane functions asa guide rail. Stationary member 530 is abutted against and secured byadhesive or otherwise to a raised, C-shaped locating feature 114 definedon the stator support surface 111 of frame 110. The slide member 568reciprocates in the Z direction along the interior of the C-shapedstationary member 530 while being held in fixed orientation in the XYplane by a pair of opposed sets of top-wall bearing balls 561 andbottom-wall bearing balls 562 that are situated at the opposed top andbottom inner walls of the C-shape of stationary member 530.

FIG. 5B provides a detailed cross-sectional top-view of a the guidemechanism 500. Slide member 568 also has a C-shape that faces in thesame direction and fits within the C-shape of the stationary member 530.The stationary member 530 is preferably abutted against the locatingfeature 114, which feature is a die-cast extension of frame 110, andwhich is made of a rigid material. The slide member 568, on the otherhand is preferably made of a resilient material with such as an aluminumalloy having spring-like flexure. Slide member 568 is dimensioned suchthat insertion of the springy slide member 568 into the rigid C-shape ofthe stationary member 530 applies a spring-loaded force H against theinner top and bottom sidewalls of the stationary member 530.

Four guide shafts are provided as two pairs 531-532, 533-534, eachagainst the inner sidewalls of the stationary member 530 (only pair531-532 is shown). Four additional guide shafts are provided as twopairs 563-564, 565-566, each against the outer side walls of slidemember 568. Shafts 531-532 oppose shafts 563-564. Shafts 533-534 (notshown) oppose shafts 565-566 (not shown). The bearing balls 561, 562(only balls 562 are shown) ride in grooves created between the opposedguide shaft pairs, 531-532 versus 563-564, and 533-534 versus 565-566. Aballs cage 567 confines the bottom bearing balls 562 to uniformly spacedvertical spots along the groove between opposed guide shafts 531-532versus 563-564. Another balls cage (not shown) confines the upperbearing balls 561 to uniformly spaced vertical spots along the groovebetween opposed guide shafts 533-534 versus 565-566 (not shown).

Shafts 531-532 are attached to the inner wall of the stationary member530 either by screws and/or with an appropriate resilient adhesive 535such as Loctite Black Max 380™. Shafts 563-564 are similarly attached tothe opposed outer wall of the slide member 568 either by screws and/orwith an appropriate adhesive 569 such as Loctite Black Max 380™.

Shafts 531-532 and shaft 563 are rigidly positioned by 90° corners ofrespective fixed and sliding members 530, 568. This defines the pathtraveled by slide member 568. Guide shaft 564 is resiliently positionedby adhesive 569 along the outer wall of slide member 568 so that thereis some give in the positioning of shaft 564. Balls cage 567 ispreferably made of a resilient material so there is give in itspositioning. The balls cage 567 is attached to the stationary member 530as shown to position the bearing balls 562 in the groove opposed guideshafts 531-532 versus 563-564.

Referring again to FIG. 5A, during times when a tape cartridge 180 isnot inserted in the tape drive, it is preferable to confine the movementof the carriage 160 so as to avoid damage from shock. A spring loadedlatch 115 is provided on frame 110 for sliding over voice-coil 140 andpreventing its movement at the time a tape cartridge 180 is withdrawnfrom the drive. Appropriate electrical drive signals are supplied to thevoice-coil motor prior to withdrawal to park the voice-coil 140 safelyinside the confines of magnetic gap 125 prior to withdrawal. A stopfeature 116 on the frame 110 opposes the spring force of the springloaded latch 115 and places a first end 115a of the latch 115 over thecarriage 160 to block movement of the carriage. When a tape cartridge180 is inserted into the drive, the tape cartridge 180 engages a secondend 115b of the latch 115 and rotates the first end 115a of the latch115 away from the reciprocating path of the carriage 160.

The above disclosure is to be taken as illustrative of the invention,not as limiting its scope or spirit. Numerous modifications andvariations will become apparent to those skilled in the art afterstudying the above disclosure. By way of example, although the "dipstick" portion 167a, 467a or 467a' of the respective LVDT, Hall andoptical position detectors of FIGS. 1, 4A and 4B are shown to beconveniently located centrally in the hollow of the voice coil forreciprocating into an opposed central hollow of the stator, otherpositionings can be used. In the case of the optical position detector(FIG. 4B) for example, an optical scale pattern can be applied (e.g.,painted on) to an inner or outer wall of the voice coil and detected assuch when it reciprocates with the voice coil into the magnetic gap 125either by an optical illuminator/detector positioned below permanentmagnets 126 or through a hole bored through one or the other of theinner and outer pole pieces 124, 127. The dip stick can be positionedelsewhere on the carriage 160 for moving in unison with the carriagerelative to the frame 110 and a position detector fixed portion that isattached to the frame 110.

Given the above disclosure of general concepts and specific embodiments,the scope of protection sought is to be defined by the claims appendedhereto.

What is claimed is:
 1. A head positioning apparatus for positioning amagnetic head in a multi-track tape drive, said apparatus comprising:(a)a voice-coil having a hollow defined in an interior portion thereof; (b)a stator assembly for generating a flux field passing through thevoice-coil; (c) a movable carriage supporting the magnetic head and thevoice coil; (d) a guide rail operatively coupled to the movable carriagefor guiding the movable carriage along a predefined guide path thatcarries the head to a desired track position of a supplied multi-tracktape and carries the voice-coil through the flux field generated by thestator assembly; (e) carriage position measuring means for measuring theposition of the carriage relative to a reference frame; wherein thecarriage position measuring means includes: (e.1) a permanent magnetcoupled to the carriage so as to move with the carriage, said magnetbeing situated within the hollow of the voice-coil; and (e.2) stationarydetecting means for detecting the position of the magnet relative to thestationary detecting means, the stationary detecting means beingsituated in a region traversed by the hollow of the voice-coil.
 2. Ahead positioning apparatus according to claim 1 further comprising:(f)voice-coil drive means for driving a current through the voice-coil toapply a motive force to the voice-coil and thereby propel the attachedcarriage and head to a desired position, the voice-coil drive meansincluding: (f.1) a nominal track positioner subsystem for moving thehead to a desired one of predefined nominal track positions; and (f.2) aclosed-loop servo subsystem for moving the head into fine alignment withservo signals of the desired track.
 3. A head positioning apparatusaccording to claim 2 further comprising:limit-indicating means,operatively coupled to the carriage and to the stator assembly, fordetecting and indicating a condition wherein a separation between thecarriage and the stator assembly is equal to or less than a predefinedlimit distance; said voice-coil drive means being responsive to thelimit-indicating means, for inhibiting movement of the carriage towardthe stator assembly if the separation between the carriage and thestator assembly is indicated to be equal to or less than the predefinedlimit distance.
 4. A head positioning apparatus according to claim 2further comprising:(g) a flexible cable having one end coupled to themovable carriage for conducting said voice coil drive current to thevoice-coil.
 5. A head positioning apparatus according to claim 1 furthercomprising:(h) limit-indicating means, attached to the carriage andoperatively coupled to the stator assembly, for detecting a conditionwherein a separation between the carriage and the stator assembly isequal to or less than a predefined limit distance and for outputting alimit-indicating signal representative of said condition; wherein theflexible cable conducts said limit-indicating signal to the voice-coildrive means; and wherein the voice-coil drive means is responsive to thelimit-indicating signal, for inhibiting movement of the carriage towardthe stator assembly if the separation between the carriage and thestator assembly is indicated to be equal to or less than the predefinedlimit distance.
 6. A head positioning apparatus according to claim 4wherein said carriage position measuring means outputs a positionmeasurement signal that is subject to error due to temperaturevariation, said apparatus further comprising:(h) temperature measurementmeans, attached to the carriage, for measuring a temperature of themoveable carriage and for generating a temperature indicating signal inresponse to the measured temperature; and (i) combining means forcombining the temperature indicating signal with the positionmeasurement signal to produce therefrom a temperature-compensatedposition indicating signal; wherein the flexible cable conducts saidtemperature indicating signal from the temperature measurement means tothe combining means.
 7. A head positioning apparatus according to claim6 wherein said magnetic head converts magnetic signals pre-recorded on asupplied multi-track tape into electrical signals, and wherein theflexible cable conducts the head-produced electrical signals.
 8. A headpositioning apparatus according to claim 2 wherein said nominal trackpositioner subsystem includes a digital storage means for defining saidpredefined nominal track positions.
 9. A head positioning apparatusaccording to claim 1 wherein the stationary means has a passagewaydefined therethrough and aligned with the magnet for permitting freemovement of the magnet through the passageway.
 10. A head positioningapparatus according to claim 9 wherein each of the voice-coil and thestationary detecting means is cylindrical in shape.
 11. A headpositioning apparatus according to claim 10 wherein the stator assemblyis cylindrical in shape and includes a plurality of permanent magnetsdisposed symmetrically about an interior portion of the cylindricalshape of the stator assembly.
 12. A head positioning apparatus accordingto claim 11wherein the stator assembly includes a inner pole piecesurrounding the magnet and shielding the magnetic head from magneticfields generated by the magnet.
 13. A head positioning apparatusaccording to claim 1wherein the stationary detecting means includesfirst and second magnetic flux detectors positioned to respectivelyoppose north and south poles of the permanent magnet for detecting thecomparative strengths of the respective magnetic flux fields coupledfrom respective ones of the north and south poles to the first andsecond magnetic flux detectors.
 14. A head positioning apparatusaccording to claim 13wherein the carriage position measuring meansoutputs a position indicating signal V_(P) of the form:

    V.sub.P =(V.sub.A -V.sub.B)/(V.sub.A +V.sub.B)             (Eq. 1)

where V_(A) and V_(B) represent respective flux strength measurements ofthe first and second magnetic flux detectors.
 15. A head positioningapparatus according to claim 13wherein one or more of said magnetic fluxdetectors is a Hall effect device.
 16. A head positioning apparatusaccording to claim 1 wherein said carriage position measuring meansoutputs a position measurement signal that is subject to error due totemperature variation, said apparatus further comprising:(f) temperaturecorrection means for measuring temperature, generating a temperaturecorrection signal in response to the measured temperature, and combiningthe temperature correction signal with the position measurement signalto produce therefrom a temperature-compensated position indicatingsignal; and (g) voice-coil drive means, responsive to thetemperature-compensated position indicating signal, for driving acurrent through the voice-coil to apply a motive force to the voice-coiland thereby propel the attached carriage and head to a desired position.17. A head positioning apparatus according to claim 16 wherein saidtemperature correction means includes:temperature measurement means,attached to the carriage, for measuring temperature and for generating atemperature indicating signal in response to the measured temperature;wherein the temperature measurement means is positioned operatively nearto the carriage position measuring means for measuring a temperature ofthe carriage position measuring means.
 18. A head positioning apparatusaccording to claim 1 wherein said movable carriage includes:guide-railengaging means for operatively engaging with the guide rail means suchthat the movable carriage will be guided along the predefined guidepath; wherein the guide-rail engaging means is positioned substantiallyat a center of gravity of the movable carriage.
 19. A head positioningapparatus according to claim 18 wherein:the guide rail means includes afixed guide rail; the guide-rail engaging means includes low-frictionmeans for riding along the fixed guide rail with substantially smallfriction to thereby minimize a corresponding friction loading on thevoice-coil when the voice coil propels the carriage; and thelow-friction means includes a first side portion and an opposed secondside portion each engaging with the fixed guide rail, said first andsecond side portions being symmetrically disposed relative to the centerof gravity of the carriage.
 20. A head positioning apparatus accordingto claim 19 wherein the low-friction means includes a first plurality oflow-friction roller wheels symmetrically engaged to the fixed guiderail.
 21. A head positioning apparatus according to claim 20 wherein:theguide rail means includes a movable guide rail; and the low-frictionmeans includes a second plurality of low-friction roller wheelssymmetrically engaged to the movable guide rail.
 22. A head positioningapparatus according to claim 19 wherein:the guide rail means includes astationary member having a C-shaped first section; and the low-frictionmeans includes a slide member having a C-shaped second section fittingsymmetrically within the C-shaped first section.
 23. A head positioningapparatus according to claim 22 wherein the C-shaped second section ofthe slide member applies resilient pressure against opposed inner wallsof the C-shaped first section.
 24. A head positioning apparatusaccording to claim 23 comprising a plurality of bearing balls disposedbetween the inner walls of the C-shaped first section and opposing outerwalls of the C-shaped second section.
 25. A head positioning apparatusaccording to claim 1 further comprising:(f) a substantially rigid frameon which said guide rail is positioned, said frame having a alignmentfeature formed thereon for engaging with the stator assembly andaligning the stator assembly relative to the guide rail means so as todefine a prespecified spatial orientation between the stator assemblyand the guide rail means.
 26. A head positioning apparatus according toclaim 25 wherein:the alignment feature is an integral part of the frame;and the stator assembly has a side portion abutted against the alignmentfeature and a base portion adhesively fastened to the frame.
 27. A headpositioning apparatus according to claim 25 wherein the guide rail meanscomprises:a fixed guide rail fixedly attached to the frame; and amovable guide rail, resiliently biased to move toward the fixed guiderail;and wherein the carriage includes: low-friction means for ridingbetween the fixed guide rail and the movable guide rail withsubstantially small friction to thereby minimize a correspondingfriction loading on the voice-coil when the voice coil propels thecarriage and to thereby define said guide path.
 28. A head positioningapparatus according to claim 1 further comprising:(f) latch means,positioned to operatively engage with said movable carriage and to bedriven by a supplied tape cartridge, for allowing movement of thecarriage when a tape cartridge supplied, and for inhibiting movement ofthe carriage when a tape cartridge is not supplied.
 29. A headpositioning apparatus according to claim 1 wherein said carriage iscomposed of a rigid material.
 30. A voice-coil based positioning systemcomprising:(a) a movable voice-coil having a tubular shape, where saidtubular shape includes inner and outer bounds; (b) a stator forgenerating a flux field passing through the voice-coil; (c) voice-coildrive means for delivering a current to the voice-coil to therebygenerate a magnetic motive force that moves the voice-coil; and (d)position determining means for determining the position of the movablevoice-coil, said position determining means having a movable firstposition determining means and a stationary second position determiningmeans, (d.1) wherein the movable first position determining meansincludes a permanent magnet that is coupled to the movable voice-coilfor moving with the voice-coil, (d.2) wherein the movable first positiondetermining means is positioned within the inner bounds of the tubularshape of the voice-coil, (d.3) wherein the stationary second positiondetermining means is positioned within a region that is bounded by apath traveled by the inner bounds of the tubular shape of thevoice-coil; and (d.4) wherein the stationary second position determiningmeans is operatively coupled to the movable first position determiningmeans for detecting the position of the movable voice-coil relative tothe stationary second position determining means.
 31. A voice-coil basedpositioning system according to claim 30 wherein said positiondetermining means outputs a position measurement signal that is subjectto error due to temperature variation, said positioning system furthercomprising:(h) temperature measurement means, operatively coupled to themovable first position determining means, for measuring a temperature ofthe movable first position determining means and for generating atemperature indicating signal in response to the measured temperature;and (i) combining means for combining the temperature indicating signalwith the position measurement signal to produce therefrom atemperature-compensated position indicating signal.
 32. A voice-coilbased positioning system according to claim 31wherein said voice-coil iscoupled to a movable load that is to be moved to one of a plurality ofpredefined nominal load positions; wherein the combining means producesthe temperature-compensated position indicating signal such that theposition indicating signal indicates the position of the load; andwherein said voice-coil drive means comprises: (c.1) first input meansfor receiving a nominal load-position signal representing a desirednominal load position; (c.2) second input means for receiving saidtemperature-compensated position indicating signal from the combiningmeans; and (c.3) servo means for adjusting the magnitude of the voicecoil driving current so as to shift the measured load positionrepresented by the temperature-compensated position indicating signaltowards convergence with the desired load position represented by thenominal load-position signal.
 33. A voice-coil based positioning systemaccording to claim 32wherein said combining means includes a digitalmemory device having a plurality of addressable memory cells eachstoring a data word, wherein the digital memory device has an addressinput port and a data output port, wherein the address input port isresponsive to the combination of the temperature indicating signal andthe position measurement signal, and wherein the data words stored inthe digital memory device are predefined to produce thetemperature-compensated position indicating signal at said data outputport.
 34. A voice-coil based positioning system according to claim 30wherein:the movable first position determining means is located withinan interior hollow defined by inner bounds of the tubular shape of thevoice-coil; and the stationary second position determining means ispositioned within to a region traversed by the interior hollow of thevoice coil.
 35. A voice-coil based positioning system according to claim34 wherein the stator includes:a U-shaped magnetic yoke through whichflows the magnetic flux of the stator-generated flux field, the U-shapedmagnetic yoke having an inner pole piece positioned in the regiontraversed by the interior hollow of the voice coil, and the U-shapedmagnetic yoke further having an outer pole piece positioned outside theregion traversed by the outer bounds of the tubular shape of thevoice-coil as the voice coil moves.
 36. A voice-coil based positioningsystem according to claim 35 wherein:the inner pole piece has a tubularshape surrounded by the tubular shape of the voice coil.
 37. Avoice-coil based positioning system according to claim 36 wherein:theouter pole piece has a tubular shape surrounding the tubular shape ofthe voice coil.
 38. A voice-coil based positioning system according toclaim 37 wherein the stator further includes:a plurality of magnetsdistributed about the tubular shape of the voice coil for distributingsaid flux field uniformly about the voice-coil.
 39. A voice-coil basedpositioning system according to claim 34 further comprising:low-frictionguide means, operatively coupled to the voice-coil, for guiding thevoice coil along a guide path defined by a guide rail, said low-frictionguide means riding along the guide rail with substantially smallfriction to thereby minimize a corresponding friction loading on thevoice-coil as the voice coil moves along the guide path defined by saidguide rail.
 40. A voice-coil based positioning system comprising:(a) amovable voice-coil having a tubular shape; (b) a stator assembly forgenerating a flux field passing through the voice-coil; (c) voice-coildrive means for driving a current through the voice-coil to therebyapply a motive force to the voice-coil and move the voice-coil; (d) amovable magnet coupled to the movable voice-coil for moving with thevoice-coil, said movable magnet being positioned within the tubularshape of the voice-coil; (e) magnetic field sensing means positionedwithin a region bounded by a path of travel of the tubular shape of thevoice-coil, the magnetic field sensing means having a Hall-effect devicemagnetically coupled to the movable magnet for detecting the position ofthe movable magnet relative to the magnetic field sensing means.
 41. Amethod for making and calibrating a positioning system comprising thesteps of:(a) providing a voice coil reciprocally disposed in a magneticstator structure, the voice coil moving relative to the stator structurein response to the application of electrical current through the voicecoil; (b) providing a programmable measurement means for measuringmovement of the voice coil relative to the stator structure, saidmeasurement means including a permanent magnet coupled to the voicecoil, the output of the measurement means being defined by aprogrammable table look-up means included in the programmablemeasurement means; (c) removably attaching a precision measurement meansto the voice coil for precisely measuring the position of the voice coilrelative to the stator structure; (d) applying current to the voice coilto move the voice coil to a precision position as determined by theprecision measurement means; and (e) programming the programmable tablelook-up means to output a signal representing said position while saidcurrent is applied to the voice coil.
 42. A load positioning apparatusfor providing coarse and fine positioning of a load, said apparatuscomprising:(a) a voice-coil having a hollow defined in an interiorportion thereof; (b) a stator assembly for generating a flux fieldpassing through the voice-coil; (c) a movable carriage supporting thevoice coil and further adapted for supporting the load; (d) a guide railoperatively coupled to the movable carriage for guiding the movablecarriage along a predefined guide path that allows the carriage to carrythe load to predefined positions and that further carries the voice-coilthrough the flux field generated by the stator assembly; (e) a carriageposition measuring mechanism for measuring the position of the carriagerelative to a reference frame and outputting a corresponding measurementsignal; wherein the carriage position measuring mechanism includes:(e.1) a first position measuring part coupled to the carriage so as tomove with the carriage, said first position measuring part having apermanent magnet situated within the hollow of the voice-coil; and (e.2)a second position measuring part for magnetically detecting the positionof the permanent magnet relative to the second position measuring part,the second position measuring part being situated in a region traversedby the hollow of the voice-coil; and (f) a coil driving circuit that isresponsive to the measurement signal of the carriage position measuringmechanism and to supplied, coarse and fine position-control signals, thecoil driving circuit outputting a responsive drive current to thevoice-coil for respective coarse and fine positioning of said load. 43.A load positioning apparatus according to claim 42 wherein saidpermanent magnet includes Neodymium.
 44. A load positioning apparatusaccording to claim 42 wherein said permanent magnet is of a low-weight,high-field strength formulation such as Neodymium.
 45. A loadpositioning apparatus according to claim 42 wherein said second positionmeasuring part includes a Hall-effect device for magnetically detectinga strength of magnetic field radiated by the permanent magnet.
 46. Aload positioning apparatus according to claim 42 wherein said secondposition measuring part includes a pair of opposed Hall-effect detectorsfor magnetically detecting strengths of opposingly-directed magneticfields radiated by the permanent magnet.
 47. A load positioningapparatus according to claim 46 wherein said carriage position measuringmechanism further includes:(e.3) signal means for producing saidmeasurement signal as a function of a difference between respective fluxstrength detections made respectively by the opposed Hall-effectdetectors.
 48. A load positioning apparatus according to claim 47wherein:(e.3a) said signal means is further for producing saidmeasurement signal as a function of a reciprocal of a sum of therespective flux strength detections made respectively by the opposedHall-effect detectors.
 49. A load positioning apparatus according toclaim 42 further comprising:(g) a magnetic shield surrounding saidpermanent magnet and said second position measuring part.
 50. A loadpositioning apparatus according to claim 42 further comprising:(g) anonmagnetic support bracket extending from the carriage to the permanentmagnet and supporting the permanent magnet; and (h) a temperaturemeasuring device operatively coupled to the support bracket formeasuring the temperature of the support bracket and for producing acorresponding temperature indicating signal; wherein the carriageposition measuring mechanism is responsive to the temperature indicatingsignal for compensating for thermal expansion of the support bracket.51. A load positioning apparatus according to claim 50 wherein thecarriage position measuring mechanism includes a calibratable lookupmeans for defining the measurement signal as a function of saidtemperature indicating signal and as a function of said magneticdetecting by the second position measuring part.